Two-Dimensional Models of Poroelastically-Controlled
Earthquake Triggering


Permeability is the primary control on the nature of fluid flow within mid-ocean ridge hydrothermal systems. This parameter determines the intensity of hydrothermal convection, the spatial form of circulation patterns, and the location and extent of downflow and upflow zones. Permeability plays a key role mediating many subseafloor processes, including fluxes of heat and chemicals, geochemical alteration, and primary production by chemosynthesizing microorganisms. Despite this importance, permeability is the most poorly constrained of the hydrological parameters in mid-ocean ridge settings.

I have developed a new technique for constraining the permeability structure in these systems using numerical models of poroelastic fluid flow. These models are calibrated using results from seismic studies that show spatial patterns in the tidal triggering of mid-ocean ridge earthquakes. The spatial triggering patterns likely result from pore pressure transients, which are generated by tidal forces and influenced by permeability structure. These pressure transients diffuse through the crust creating "waves" of enhanced seismicity by lowering normal stresses on fault surfaces. Relative phase lags in stresses associated with these pressure transients can be modeled to approximate the stresses inferred from earthquake data. These models provide constraints on the permeability structure of young oceanic crust.

Preliminary results using a microearthquake catalog from the East Pacific Rise show that the phases of volumetric stresses inferred from the earthquake data can be approximated using realistic permeability values and simple permeability distributions. These models predict that the average permeability in the upper 1.5 km of crust in the along-axis direction is about 10-12 m2, which is consistent with estimates based on heat flow data. Future work using forward modeling techniques will provide new insights into the permeability structure of seafloor hydrothermal systems.

Animations

Animation showing pressure perturbations within the model domain resulting from ocean tidal loading and Earth tide stresses. Pressure perturbations are dominated by ocean tidal loading:

Animation showing effective stress perturbations for the poroelastic frame within the model domain. Effective stresses are dominated by the Earth tides, but are strongly mediated by pore pressure perturbations: